As known, the bicycle is a mechanical device moved by muscle power, hence one of the main requirements to be satisfied in the field of bicycles, particularly racing bicycles, is to allow the best possible exploitation of such a muscle power.
In particular, in the field of racing bicycles it is attempted to meet this requirement by increasing the number of gear ratios.
A conventional motion transmission system comprises, in the rear part of the bicycle, a hub associated with the rear wheel of the bicycle and provided with a body, in the jargon known as “freewheel body,” capable of rotating idly with respect to the hub in one direction of rotation and of pushing the hub into rotation in the opposite direction.
The freewheel body is in general a cylindrical body having, on the radially outer surface thereof, a plurality of longitudinal grooves configured to engage the sprockets of a sprocket assembly.
The sprockets in turn are engaged with, and pushed in rotation by, a chain which receives the movement from at least one toothed front sprocket associated with one of the two crank arms coupled with the shaft of the bottom bracket assembly of the bicycle, and driven by a rider.
The sprockets are generally made of an extremely strong material (like steel or titanium) to withstand the wearing generated by the sliding of the chain and the loads which they are subjected to during operation. The sprockets can comprise a radially inner surface provided with grooves configured to engage the grooves of the freewheel body. Alternatively, the sprockets can be supported by support elements, often made from light material (like aluminium), having a radially inner surface provided with grooves configured to engage the grooves of the freewheel body. A support element can also support more than one sprocket.
The sprockets of the sprocket assembly are spaced apart through suitable spacers. In general, each spacer is arranged between two adjacent sprockets or between two adjacent support elements, if provided. In the case in which the support elements support two sprockets arranged on opposite sides, they also act as spacers.
The spacers can comprise a radially inner surface provided with groves configured to engage the grooves of the freewheel body.
In the field of racing bicycles, sprocket assemblies are known having nine, ten, or even eleven sprockets.
The sprocket having the smallest diameter of the sprocket assembly has a radially outer toothing (configured to engage the chain) with, typically, eleven or twelve teeth.
The Applicant has felt the need to add, to a sprocket assembly of the type described above, an additional sprocket having a smaller number of teeth with respect to the number of teeth of the sprocket having the smallest diameter of the sprocket assembly, so as to provide the cyclist with a further gear ratio characterised by being extremely long, thus very suitable for flat terrain, downhill and also for sprint finishes in cycling competitions.
Hereinafter, the aforementioned additional sprocket is also indicated as “last sprocket”, whereas the aforementioned sprocket having a smaller diameter is also indicated as “second-last sprocket”.
The Applicant has observed that the provision of an additional sprocket in a sprocket assembly requires the provision of an additional spacer arranged between such a sprocket and the sprocket having a larger diameter which is immediately adjacent thereto, such a spacer being configured to be fitted onto a free end portion of the freewheel body.
Hereinafter, the aforementioned additional spacer is also indicated as “last spacer”.
The Applicant has observed that, in order to ensure that the additional spacer has a sufficient structural strength, since the inner diameter of the additional spacer is imposed by the outer radial size of the freewheel body, the additional spacer should have an outer diameter that in certain cases can be greater than the diameter defined in the additional sprocket at the bottom of the throats of the toothing.
The Applicant has, however, observed that in this case it would not be possible to achieve a correct meshing of the chain on the teeth of the additional sprocket.